The present invention relates to a method of assigning a seismic trace, for instance in dip-moveout correction, normally referred to as "DMO".
In seismic exploration, acoustic signals produced by a seismic source travel downwardly into the earth and are reflected back to a number of seismic receivers, such as geophones for use on land or hydrophones for seismic exploration below the sea. The digitally recorded signals received by the receivers are normally referred to as traces and are processed in order to yield information about the nature of the earth below the area being investigated. For instance, these signals carry information indicating the structure of reflective layers such as boundaries between different types of rocks.
The procedure of converting recorded traces into a subsurface image is typically divided into several steps, each producing an intermediate result which may be useful. Ideally all the reflected signals are transformed (or "migrated") to their actual subsurface location, and are there combined, by summation, with all data corresponding to the same location. This procedure may in principle be performed in a single step, referred to as "prestack migration" by those skilled in the art. However, in order to facilitate parameter selection and reduce computational requirements, this procedure is usually subdivided into four steps.
Firstly, two corrections are made to eliminate the effects of source-receiver separation (or offset): one a velocity dependent correction known as normal moveout (NMO), which assumes reflections occur at horizontal interfaces; the other a velocity independent correction known as dip moveout (DMO), which compensates for the mispositioning due to any inclination (or dip) of the reflecting interfaces. The theory of dip moveout is generally based on constant velocity, but it is sufficiently accurate for most cases where velocity varies. Application of NMO and DMO produces traces which simulate the recording of a survey with the source and receiver at the same location (zero offset traces), and permits the summation (or stacking) of traces with the same or similar locations, to produce the "stack". As well as reducing the number of traces for subsequent processing this step improves the signal-to-noise ratio of the data. Finally, reflectors are moved to their correct positions by a zero offset migration of the stack.
In the case of 3D seismic data, in which the survey has been conducted with the sources and receivers arranged to cover an area of the surface and so obtain data from a 3-dimensional portion of the earth, the traces are collected into geometric cells (or bins) which make up a regular grid, either at the surface or at some reference plane defined for processing purposes. The stack is obtained by summing traces which fall within the same cell, to generate a single trace for each grid location The location of such traces is partly determined by the arrangement of sources and receivers, but may also be affected by the location of traces generated by the dip moveout step, prior to stacking.